Aircraft fuel naturally contains some dissolved gas, typically air, and therefore typically contains some dissolved oxygen. The amount of oxygen in fuel decreases with pressure. Therefore, at cruising altitude (i.e., at low ambient pressure), oxygen is degassed from the fuel. From a safety standpoint, it is desirable to have fuel or fuel-rich environments contained in an inert atmosphere. Thus, the release of oxygen from fuel in such an environment is highly undesirable.
Also, evolved gas from the fuel may increase the risk of air pockets forming in the fuel system of the aircraft. Some aircraft fuel tank arrangements use gravity feed systems including siphons to transfer fuel within the fuel system. Air pockets present in the fuel tank system may act to disrupt the siphon effect in gravity feed systems, the pressure head possibly being insufficient to push the air down the pipes.
One known technology for estimating oxygen concentration uses electrochemical detection. However, most commonly used electrolytes are not suitable for extreme operating temperatures. In particular, they are not suitable for the low temperatures encountered in aviation applications.
More recently, oxygen monitors for aircraft fuel tanks have been developed in which the oxygen concentration is monitored by means of a probe containing a luminescent substance. Oxygen acts to quench the luminescence of the luminescent substance and therefore the concentration of oxygen can been derived from measurements of the light emitted from the luminescent substance.
WO 03/046422 describes one such system in which the oxygen concentration in an aircraft fuel tank is monitored by means of a monitor containing a fluorescent ruthenium complex.
US 2006/0171845A1 discloses the use of platinum (II) tetrakis(pentafluorophenyl)porphyrin as a fluorescent compound held within an amorphous fluorinated polymer matrix for the detection of oxygen in an aircraft fuel tank.
In order to be suitable for use in an aircraft fuel tank a monitor must be capable of withstanding the low temperatures, for example, −20° C. to which the fuel tanks are exposed when the aircraft is in flight. Furthermore, for obvious reasons it is desirable that the monitor be reliable and have a lifetime which is numbered in years rather than months. Furthermore, the materials used in the monitor must be compatible with aviation fuel.
The present inventors have found that the luminescence of some substances is effectively switched off by contact with the aviation fuel and therefore the sensing mechanism becomes inoperative. The reason for that is not known but it is possible that aviation fuel contains certain additives which act to prevent the luminescence. Furthermore, luminescent materials held in a polymer matrix can over time leach out from the polymer into the aviation fuel, thereby reducing the effectiveness of the monitor.